Stabilization of liquid sulfur trioxide and oleums



Unite tates STABILIZATION F Rudolph Pick, Elizabeth, N. 1., assignor-to 'E. I." duPont de 'Nemours-and Company, Wilmington, DeL, a cor poration of Delaware NoDrawing. Application January .3, 1956 Serial No. 556,814

.8 Claims. .(Cl. 123-174) "neat The polymeric form melting at 62 C., alpha S0 is the stable form under ordinary conditions. Upon freezing or even after standing at room temperature a short time the liquidgamma formtMvP. -17 C.) ordinarily changes to the solid forms.

This transition of liquid sulfur trioxide to solid polymeric forms is extremely undesirable from the standpoint of ease in handling andusage. In most instances, prior to usage the solidified sulfur trioxide must, be remelted by heating ,to temperatures .up to about 100 C., thereby developing dangerous high pressures in some instances.

High strength oleums of free sulfur trioxide strengths upward of about 80% tend to polymerize in a similar fashion. The degree of polymerization depends chiefly upon the sulfur trioxide strength of the oleum and the temperatures at which the material is stored. While polymerization will not proceed sufliciently far in some of these .oleums to give a completely solid product, the degree of polymerization increases on the passage of time so that the ultimate mass will ordinarily contain solid forms of 80;, in supension in the oleum. This polymerization of sulfur trioxide in high strength oleums is also highly undesirable.

I have discovered that liquid sulfur trioxide and oleum of 80;, strength such that S0 polymers tend to form can be stabilized against S0 polymerization by incorporating therein a minor amount of one or more silicon halides, and then heating the product for several hours in the range of about 50 C.l00 C. The preferred heat treatment is for at least six hours at about 80 C.

The amount of inhibiting agent to be incorporated in the S0,; or oleum will vary with the use for which the SO -containing product is intended and with the conditions it will be expected to meet. The stabilizing infiuence of these inhibiting agents is proportional to the amount present. In general, it usually is desirable to use only sufficient amount to stabilize the product against substantial polymerication of S0 although relatively larger amounts of inhibiting agents can be used where the presence of an excess is not objectionable. In most cases it is desirable to use no more than about 10% by weight of silicon halide, with from 0.2 to 5% based on the free S0 content being the preferred range. The higher percentages ordinarily are employed to stabilize the oleums.

The preferred compositions of this invention consist of liquid sulfur trioxide or oleum containing not more than about 0.1% of water, which materials contain from 0.2 to 5% by weight of the stabilizing agent. Oleum containing not more than about 0.1% of water is, of course, oleum of S0 strength not less than about 99.4%

and--ssulfuric acid content of not more :than 0.56%. These materials high in a sulfur trioxide :can :be stabilized readily-soas'to resist polymerization even after-storage for long periods of time.

The stabilizing agent can be added directly to. the liquid sulfur trioxide or oleum to be stabilized followed by thorough agitation of the liquid. Alternatively, in the:C3S6' of stabilizing pure liquid sulfur trioxide, vS0 vapor-can be condensed in a closed vessel containing the desired amount of stabilizing agent.

All silicon halides, that'is the silicon tetrahalides and the: disilicon hexahalides, 'such as silicon :tetrafluoride, disiliconhexaiodide, silicon tetrabrornide, disilicon hexachloride, silicon tetraiodide and the like are effective stabilizing agents'in the practice of my invention. "However, silicon tetrachloride is by far themost preferred stabilizing agent and by no means-the equivalentwof the above-mentioned silicon halides from. the standpoint of ease of -usage and properties of the stabilized S0 .produced.

Materials stabilized in accordance with my method do not-polymerize to any appreciable extent. They remain substantially in the liquid gamma form at room temperature. Furthermore, .the stabilization inhibitor functions as anantifreeze, lowering the freezing-point substantially inmost instances. Even after freezing by being cooled to alow temperature, they remelt-rapidlyaupon reheat-ing-io'about 35 .C. -Therefore, these'products can be used, directly in commercial application without first putting them through a tedious-remeltingiprocess. inorderthat .theinvention can be better-understood, the following examples in addition :to those/set'forth above are given:

Example 1 One percent by weight of silicon tetrachloride is added with agitation to anhydrous liquid gamma sulfur trioxide. The liquid sulfur trioxide is then heated for 16 hours at C. The thusly stabilized liquid sulfur trioxide remains liquid upon cooling to temperatures close to 0 C., indicating the definite antifreeze function of the stabilization inhibitor. Upon cooling the mixture to 0 C., it freezes; upon reheating to 35 C., the stabilized liquid sulfur trioxide remelts readily. After standing at room temperature for several days, the composition exhibits no noticeable polymerization.

Example 2 Three percent by weight of silicon tetrafluoride is added with agitation to anhydrous liquid gamma sulfur trioxide. The mixture is then heated at 60 C. for 24 hours. After standing at room temperature for 72 hours, the thusly stabilized liquid sulfur trioxide shows no tendency to polymerize and solidify.

Example 3 Five percent by weight of disilicon hexabromide is added with agitation to oleum of 99.5% S0 strength. The mixture is then heated at 70 C. for 8 hours.

After freezing the thusly stab'dized liquid sulfur trioxide by cooling to a low temperature, it is found to remelt readily upon reheating to about 35 C. After standing at room temperature for several days, the mixture shows no tendency to solidify.

Example 4 Three percent by weight of silicon tetraiodide is added with. agitation to oleum of 99.7% 80;, strength. .The mixture is then heated for 8 hours at 60 C. Uponstandr ing for several days at room temperature, the S0 comanemone 7 position remains liquid showing no tendency toward poly- 80;; polymers tend to form comprising incorporating therein about 0.2 to 5% by weight based on 80;; content of a binary silicon halide selected from a group consisting of silicon tetrahalides and disilicon hexahalides and then heating for several hours at a tem'perature in the range of 50 C. to 100 C. i g

2. The method of stabilizing liquid sulfur trioxide containing not more than 0.1% of water against 80;; polymerization comprising incorporating therein about 0.2 to 5% by weight based on S content of a'binar'ysilicon halide selected from a group consisting of silicon tetrahalides and disilicon hexahalides. q A

3. The method of stabilizing liquid sulfur tri'oxide against S0 polymerization comprising incorporating therein about 0.2 to by weightbased on S0 content of a binary silicon halide selected from a group consisting of silicon tetrahalides and disilicon hexahalides.-

4. A stabilized S0 composition comprising binary sili 3o con'halides selected-from the group consisting of silicon tetrahalides and disilicon hexahalides, in an amount up to about 'by weight based on the content; and,

5. A composition in accordance with claim 4 wherein a said binary silicon halide is present in an amount of about 0.2 to 5% by weight of the S0 content.

6. A composition comprising liquid sulfur trioxide containing not more than about 0.1% of water and not more than 10% by weight based on-SO content of silicon tetrachloride. g I V I 7. 'Ihe methodof stabilizing against-S0 polymeriza tion the material selected from the group consisting of sulfur trioxide and oleum ofSO strength such thatSOg polymers tend to form comprising incorporating therein a binary silicon halide selected from the group consisting of silicon tetrahalides and disilicon-hexahalides in an amount up to 10% by weight based on S0 content.

8; The method of stabilizing against S0 polymerization the material selected from the group consisting of sulfur trioxide and oleum of S0 strength such that S0 polymers tend to form comprising incorporating. therein then heating theresulting composition for several hours i at a temperature in therange of 50 to C..

References Cited in the file of this patent 'UNITED STATES PATENTS 2,716,593 Litant et a1. Aug. 30, 1955 FOREIGN PATENTS 669,214 Great Britain a Mar. 26, 1952 OTHER REFERENCES Manor; Vol. "6, Comprehesive Treatise on Inorganic 35 and Theoretical Chemistry, page 965, 2nd paragraph 

1. THE METHOD OF STABILIZING AGAINST SO3 POLYMERIZATION A COMPOUND SELECTED FROM THE GROUP CONSISTING OF LIQUID SULFUR TRIOXIDE AND OLEUM OF SO3 STRENGTH SUCH THAT SO3 POLYMERS TEND TO FORM COMPRISING INCORPORATING THEREIN ABOUT 0.2 TO 5% BY WEIGHT BASED ON SO3 CONTENT OF A BINARY SILICON HALIDE SELECTED FROM A GROUP CONSISTING OF SILICON TRETRHALIDES AND DISILICON HEXAHALIDES AND THEN HEATING FOR SEVERAL HOURS AT A TEMPERATURE IN THE RANGE OF 50*C. TO 100*C. 